The present invention relates to ground fault protection for use with electrical systems and more specifically to high resistance ground fault protection methods and apparatus.
A typical power/drive system includes a converter, an inverter and a mechanical load such as a motor. The converter is typically linked to a three phase source that provides three phase AC power and converts the three phase power to DC power across positive and negative DC buses. The DC buses feed the inverter which generates three phase AC power on output lines that are provided to the load. The inverter controls the three phase AC voltages and currents to the load so that the load can be driven in a desired fashion. Cables connect the source to the converter and also connect the inverter to the load.
Power/drive systems require protection from inadvertent cable and load (e.g., motor) failures which can lead to undesirable ground faults. The root cause of cable failures is often cable insulation breakdown and therefore most ground faults occur in the cables between the power source and the converter or between the inverter and the load. When a ground fault occurs, the results can be extremely costly. For instance, ground faults often result in power interruptions, equipment failure and damage, uncoordinated system decisions with potential for overall plant interruptions, degraded or lost production and overall customer frustration.
There are generally three ways to deal with the possibility of ground faults including ungrounded systems, solidly grounded systems and high resistance systems. In a solidly grounded system, when a fault occurs the system is automatically grounded and the entire process associated with the system is halted until the fault condition is eliminated. In addition to reducing productivity when they shut down, the solidly grounded systems also have problems with excessive currents that often damage system components when faults occur.
In ungrounded systems, the system continues to operate until the system is controlled to shut down for routinely scheduled maintenance at convenient times that do not appreciably affect productivity or the system fails due to equipment damage from a second phase to ground fault or equipment failure from a restriking fault raising the voltage to unacceptable levels.
In high resistance systems, system signals are monitored to identify operating characteristics that indicate the beginnings of fault conditions and thereafter a system operator is supposed to shut down the system for repairs at a convenient time and prior to occurrence of a full fault condition.
In a three phase system, it is know that power/drive systems can operate after a single phase fault occurs but that the system cannot operate when two phases fault concurrently. Thus, from the time a first ground fault occurs, system reliability is at risk should a second fault occur. The possibility of a second fault is increased because the first fault increases the phase to ground voltage for the un-faulted phases by as much as the square root of three. Thus, notice of and timely attention to a first fault are pre-requisites for a high resistance system to operate properly.
Once a fault is detected, a power/drive system has to be shut down and an engineer has to identify the cause of or condition related to the fault and eliminate the cause or condition. Identifying fault cause can be a frustrating process, especially in three phase systems that, in some cases, may have cables that are several hundred or even thousand feet long.
Prior to the advent of high resistance fault systems, because of the need for process continuity in many facilities, ungrounded systems were most prevalent. However, experiences with multiple failures due to arcing ground faults has resulted in a change in philosophy over the use of ungrounded systems and high resistance grounded systems are quickly becoming the norm. Because of the ability to control system shut down times and to only shut down when evidence of a fault can be identified, high resistance systems are often seen as advantageous.
Known systems for detecting utility line to ground faults in a high resistance ground system sense the current in the HRG and determine if the sensed current exceeds a threshold. The sensed current is assumed to be zero sequence, a phrase meaning a component that is common to all three phases and that is at the system operating frequency (e.g., where a load is operating at 60 Hz, the zero sequence component would also be a 60 Hz component). In these systems, when either the HRG current or the zero sequence current formed by summing all three phases is not equal to zero (or is greater than some minimal threshold), the condition is identified as a fault. Here, the phase with the maximum current is selected as the fault phase.
Ground current and zero sequence component approaches have been viewed as simple and straightforward. Unfortunately, with high resistance grounded systems, these approaches are not extremely accurate for a variety of reasons. Most importantly, it has been recognized that under certain cabling and grounding conditions the ground current does not equal zero when no fault is occurring and, in some cases, can add up to zero when a fault actually occurs. In addition, it has been observed that the maximum phase current when a fault is detected is not always related to the faulting phase. These “false fault” indications are a result of complex current paths that exist in high resistance grounded systems with unsymmetrical cable coupling where most systems are uniquely configured and therefore system specific current paths can have very different affects on the ground, common mode and zero sequence components.
To deal with false fault indications, current motor/drive systems typically require customers to, after a system is configured so that system specific current paths exist, set a threshold sensitivity level for zero sequence component unbalance during a commissioning procedure where possible faults will not be reported when the unbalance is below the threshold level. For instance, in some cases the sensitivity threshold may be set to 25% of a drive rating. The sensitivity threshold is typically set by trial and error (i.e., by empirical testing). When sensitivity is too high (i.e., when the threshold is set too low), false ground faults are erroneously reported and when sensitivity is too low (i.e., when the threshold is too high), faults can be missed and it is very difficult to set sensitivity to a suitable level.
For this reason it would be advantageous to have a ground fault detection system that is more reliable than the known zero sequence component systems for use with high resistance ground fault detection systems.